Flyback power supplies are a prior art switching-type power supply. They are simple, efficient, small, lightweight and cost effective. However, at high voltages, the cost of the components installed therein can reduce this type of power supply's cost effectiveness. To show how the cost effectiveness is reduced as the line voltage increases, reference is made to FIG. 1, which shows a typical high-voltage flyback power supply 10 of the prior art.
Alternating current ("AC") main, or line, power is supplied to the power supply from an external source. Industrial applications generally utilize a main power source of four hundred and forty (440) AC volts ("VAC") . This signal is rectified by full wave rectifier 12 and filtered by filter capacitor 14. This rectification and filtering translates the 440 VAC into a nominal direct current voltage of six hundred and twenty-five (625) volts ("VDC"). While the power supply 10 must be designed to handle peak input voltages significantly higher than 625 VDC because line voltage fluctuations and transients in industrial main lines can lead to worst case maximum voltages as high as eight hundred (800) VDC. This voltage is known as the "input voltage". These voltage fluctuations and transients are quite common in industrial environments and can be caused by such occurrences as the turning on or off of a piece of machinery supplied by the AC main.
Power supply 10 also comprises clamping circuit 16, which comprises Zener diodes 22 and 24 and diode 26. In exemplary prior art high voltage power supplies 10, each Zener diode 22 and 24 typically has a zener voltage of two hundred (200) volts. Clamping circuit 16 is used to limit the sum of the reflected voltage of the primary and the voltage spike caused by the inherent leakage inductance of the primary winding 19 of the output transformer 18 to a certain voltage. This reflected voltage and voltage spike will be discussed below. Secondary winding 20 is connected to diode 28 and capacitor 30 in series circuit configuration. The output 31 of the power supply 10 is across capacitor 30.
The output 31 is connected to a sense circuit 32. The output of sense circuit 32 can be connected to the input of an isolation circuit 33. Isolation circuit 33 is only present in those power supplies 10 where output 31 needs to be isolated from the AC main. If an isolation circuit 33 is present, its output is connected to a pulse width modulation controller 34. If an isolation circuit 33 is not present, the output of the sense circuit 32 is connected to the pulse width modulation controller 34. The output of the pulse width modulation controller 34 is connected to the gate of flyback switching transistor 36, which is a n-type metal oxide semiconductor field effect transistor (MOSFET).
In prior art power supply 10, the switching transistor 36 must be rated for a very high drain-source breakdown voltage because it will be exposed to very high voltages. When the switching transistor 36 is turned on, diode 26 is reversed biased, and the line voltage from the positive direct current ("DC") rail is applied to the primary winding 19, thereby ramping up its current.
When the switching transistor 36 turns off, the voltage across the primary 19 reverses so that the transformer 18 can deliver its current to the secondary 20. This reversal of the primary 19 voltage is due to the voltage reflected from the secondary 20 when the secondary 20 is conducting current. This is known as "reflected voltage". With reference to the drain of transistor 36, this reflected voltage adds to the input voltage. The input voltage can be as high as 800 VDC, as explained earlier. The magnitude of the reflected voltage can be controlled by adjusting the winding ratio of the transformer 18, i.e., the ratio of the number of windings in the primary 19 versus the number of windings in the secondary 20. Generally, the winding ratio is selected such that the reflected voltage of the primary 19 is limited to 300 VDC.
When switching transistor 36 turns off, most of the energy in the transformer 18 is delivered to the secondary 20. However, the energy stored in the leakage inductance of the primary 19 causes a spike in the reflected voltage, which is delivered to Zener diodes 24 and 22 through diode 26. This voltage spike caused by the leakage inductance of the primary 19 results in the primary 19 voltage rising well above 300 VDC for a short period of time, thereby forward biasing the diode 26. If the reflected voltage plus the voltage spike from the leakage inductance of the primary 19 becomes high enough, Zener diodes 22 and 24 will enter their breakdown region. In general, the voltage spike caused by the leakage inductance of the primary 19 will be large enough to put Zener diodes 22 and 24 into their breakdown region. Since Zener diodes 22 and 24 each typically have a zener voltage of 200 VDC, the reflected voltage of the primary will be clamped to 400 VDC. After the primary 19 delivers the energy stored in its leakage inductance, i.e., after the voltage spike settles, the voltage across the primary 19 drops to the reflected voltage from the secondary 20. However for the short period of time when the voltage spike caused by the leakage inductance of the primary 19 occurs, the switching transistor 36 must endure very high voltages. Then, after the transformer 18 has delivered its energy to the secondary 20, the reflected voltage on the primary 19 collapses to zero.
Thus, very soon after transistor 36 turns off, the voltage at the drain of transistor 36 can be as high as 1200 VDC with respect to the reference point, which in this case is the negative power supply input rail. This is because the voltage at the drain of switching transistor, which as discussed can reach a peak of 800 VDC (the worst case input voltage) adds to the 400 VDC (the clamped spike voltage caused by the leakage inductance of the primary 19). Thus, the switching transistor 36 used in the power supply 10 of the prior art must have a drain-source breakdown voltage of greater than 1200 VDC, and preferably 1300 VDC, for proper operation and to avoid damage.
MOSFETs having such high drain-source breakdown voltages are very expensive. For example, the IRFCG20 from International Rectifier has a drain-source breakdown voltage of one thousand (1000) VDC. This product is priced at approximately three dollars. In comparison, a MOSFET having a drain-source breakdown voltage of six hundred (600) VDC such as the IRFBC20, also from International Rectifier, is priced at approximately fifty cents. Thus, there is a need for a high-voltage flyback power supply that eliminates the need for such a high cost switching transistor 36. The various embodiments of the invention of this application improve upon such prior art flyback power supplies by eliminating the need for such a high cost switching transistor, thereby reducing the cost of the power supply while maintaining the performance advantages of the prior art.